The study of the natural history of cancer in domestic animals and the interrelationship of cancer in man and animals has been limited because of the lack of adequate animal data. The Animal Neoplasm Registry in Alameda and Contra Costa Counties, California (ANR), initiated July 1963, has developed methods of systematically collecting cancer cases diagnosed by practicing veterinarians. This is the first general registry in the world of histopathologically confirmed tumors of all types diagnosed in the animal populations of a defined geographic area. The main objectives of the ANR are to collect case histories and specimens on all naturally occurring animal cancers in this geographic area and animal population, and to provide a source of cases for epidemiologic and laboratory studies of animal cancers. A population based human tumor registry, the Alameda Tumor Registry, also functions in the same area. Thus human and animal cancer cases are available for comparison from the same geographic area. From these data, the following types of studies can be carried out: (1) Characterization of animal cancers such as incidence rates, including age, sex and breed specific. (2) Etiological studies such as seeking factors or variables that may be affecting the risk or occurrence of various cancers in animals. (3) Human-animal association studies such as whether there is increased human cancer in households where an animal with cancer has lived. (4) Human-animal comparisons such as the interpretation of effects noted in animal species to what may be occurring in man. (5) Laboratory studies such as attempts to isolate oncogenic viruses from various tumors or other cancerous animal tissues.